The present invention relates to the field of ultrasonic scanning echography and it relates more particularly to an echography process and device for locating the zones of reflection of a fine ultrasonic beam directed into a workpiece through a surface thereof.
The invention finds an important application, although not exclusive, in the non destructive testing of workpieces, and in particular in testing welded or solid pieces in which the cracks form reflectors for the ultrasonic beams directed transversely to the direction of the crack.
Numerous echography processes are already known using scanning of the workpiece to be tested by means of a focussed ultrasonic beam. The principle used is generally the following:
A finely focussed ultrasonic beam is directed into the workpiece. When it meets a reflector, formed by a heterogeneity, an echo is received in return whose amplitude depends on the area of the reflector (if it is less than the section of the beam), its orientation and its reflection coefficient. The echo is collected in general by the transmitting transducer and the reflector may be located by measuring the time of flight. Thus, the contour of a reflector may be determined by selecting the echoes whose attenuation, with respect to the amplitude corresponding to an extended plane reflector having the same orientation, at the same depth, is less than a threshold, 6 dB in general.
As a general rule, the exploration of a workpiece is effected by scanning its surface in two perpendicular directions, with a constant incidence in a plane which, during the exploration, keeps the same orientation with respect to an axis of symmetry of the workpiece. These processes have the serious disadvantage of only allowing reflectors to be detected which are struck by the ultrasonic beam at an incidence not deviating too much from the perpendicular.
Reflectors may then pass unnoticed and this is a particularly serious disadvantage for non destructive testing.
It is an object of the invention to provide an improved echography process. It is a more specific object to provide a process in which the probability of failure to detect zones forming ultrasonic reflectors, such as cracks or flaws in a medium, such as a workpiece, is reduced whatever the azimuth angle of these reflectors (orientation with respect to the perpendicular to the surface of the workpiece).
To this end, there is provided an echography process in which a fine ultrasonic beam is directed into the medium through the surface of said medium and the position of the zone providing an echo for several beam positions is determined and the ultrasonic beam is successively moved along the generatrices of several cones each having a rotation symmetry and having their apex on said surface and their axis perpendicular to said surface, having advantageously the same angle at the apex.
This movement is effected so that the region to be explored receives the beam at several different azimuth angles and in that the echo data corresponding to the same zone and to different azimuth angles is combined to provide a perspective representation of the medium. A isometric projection is provided.
It can be seen that all the signals obtained during scanning of the workpiece and whose data show that they concern the same elementary volume of the piece are superimposed at the same point of the representation. This solution may be used in a simple way, for there already exist on the market numerous computer programmes for providing a perspective representation, often even allowing to rotate it on a display screen such as a CRT.
To obtain high resolution, the ultrasonic beam will generally be formed from a transducer formed from a single wafer of piezoelectric material mounted against a cushioning block so as to obtain short pulses of appropriate shape or associated with an ultrasonic focussing lens.
In general, it will be sufficient to detect, for each orientation, the presence of echoes whose amplitude is greater than the threshold (typically of -6 dB with respect to an extended reflector) and to store the coordinates of the zones giving rise to such an echo.
In a variant for providing a more elaborate representation, the amplitude of the high frequency echo signal received is digitized, which corresponds to several gray levels or to arbitrary colors. The medium is considered as consisting of volume elements each centered on a node of a three-dimensional network and all echoes corresponding to a separate one of the volume elements are summed. Such a process leads to improving the signal/noise ratio by averaging the high frequency signal. It is obviously necessary to have available a much higher memory volume, since a memory element must be made to correspond to each volume element of the medium.
The invention also aims at providing an echography apparatus capable of being constructed by the addition of simple and inexpensive components to an existing echography installation already comprising computing means, which apparatus increases the capacity of the device to detect ultrasonic reflectors in the medium whatever the azimuth angle of these reflectors. To this end, an apparatus comprises means for emitting a fine ultrasonic beam and for receiving echoes, a device for moving said means outside the medium so as to provide scanning of this latter, and circuits for computing the coordinates of the zones producing an echo greater than a predetermined threshold device is provided for imparting to said means a scanning movement formed by a rotation about an axis perpendicular to the surface while maintaining them in an orientation such that the beam penetrates into the workpiece along the axis of rotation with a preliminary constant distance of travel, and to repeat this scanning about several axes perpendicular to the surface so that each zone to be examined receives the beam at several different azimuth angles and in comprises means for combining the echoes coming from the same zone so as to provide a perspective representation of their location in the medium.
The computing means will be generally provided for computing the position of the echo sources in a coordinate system related to the workpiece from the travel time of the ultra-sounds, from the position of the axis and from the coordinates of the beam emitting means with respect to the axis.
The invention will be better understood from reading the following description of particular embodiments, given by way of non limiting examples.